Although this video is focused on Warhammer 40K, I think the techniques shown for building terrain entirely out of cardboard are worth studying.
Modeling
Making Tabletop Rivers
The LudStuff blog has a nice essay with photos on making tabletop rivers.
Poker Chip Terrain Bases
Carmen’s Fun Painty Time blog has a couple nice examples of terrain bases made from poker chips. He’s a very talented guy.
Making A Spiked Barrier – Palisade
Intructables has an article on making a “spiked barrier for a Rohan gaming table,” but this sort of palisade would be great for a huge variety of periods and scenarios.